1. Field of the Invention
The present invention relates to a thermosensitive recording composition, and a thermosensitive recording material employing the above-mentioned recording composition.
2. Discussion of Background
The thermosensitive recording material has such a structure that a thermosensitive recording layer capable of inducing color development by the application of heat thereto is provided on a support such as a sheet of paper or synthetic paper, or a resin film. A thermal printer equipped with a built-in thermal head is usually used as heat application means for inducing the color development.
The thermosensitive recording system using the above-mentioned thermosensitive recording material is advantageous over other conventional recording systems because development and image fixing steps are not necessary, recording can be readily achieved using a relatively simple apparatus, and the manufacturing cost can be reduced. The thermosensitive recording system is therefore used in various fields. For example, a thermosensitive recording label sheet is attached to the wrapping material for perishable foods and cooked foods in the field of point-of-sales (POS) system; copies of books and documents are made using the thermosensitive recording material in the copying field; the facsimile machine transmits images using the thermosensitive recording material in the communication field; and data is output on a ticket paper or receipt employing the thermosensitive recording material. In the above-mentioned wide range of applications, the thermosensitive recording material is required to have heat resistance when attached to the wrapping material for cooked food in view of heating treatment before the cooked food is served. On the other hand, it is not necessary to consider the heat resistance and the long-term preservability of the thermosensitive label attached to the wrapping material for perishable foods. Further, when the thermosensitive recording material is used as a receipt paper for money or goods, there is a demand for the preservation stability of the images recorded in the thermosensitive recording material for several years. In other words, the requirements for the thermosensitive recording material, in particular, with respect to the heat resistance and preservation stability, have expanded in line with diversified usage thereof.
The thermosensitive recording composition comprises as the main components a leuco dye that is colorless or light-colored at room temperature and a color developer such as an organic acid material capable of inducing color formation in the leuco dye by the reaction with the leuco dye upon application of heat. The thermosensitive recording layer comprising a composition prepared by adding a sensitizer to the above-mentioned leuco dye and color developer is provided on a support, thereby producing a thermosensitive recording material.
As the color developer for use in the conventional thermosensitive recording material, bisphenol A is widely used. The use of bisphenol A has been controlled in consideration of environmental protection, so that it is inevitable to develop a substitute for bisphenol A as the color developer.
It is 4,4xe2x80x2-bisphenol S that has been regarded as the substitute for bisphenol A because it is non-toxic and easily available in the same manner as in bisphenol A. However, 4,4xe2x80x2-bisphenol S has a melting point of 245xc2x0 C. or more when measured by DSC, that is relatively higher than those of other color developers, and in addition, shows considerably poor coloring sensitivity. Therefore, the use of 4,4xe2x80x2-bisphenol S as the color developer has not been put to practical use.
Many trials are made to improve the coloring sensitivity of 4,4xe2x80x2-bisphenol S by the addition thereto of a variety of sensitizers, for example, bis(p-methylbenzyl)oxalate, 4-acetotoluidide, 1,2-bis(3-methylphenoxy)ethane, and various thermofusible materials. However, the coloring sensitivity is still insufficient at the present stage.
Furthermore, when 4,4xe2x80x2-bisphenol S is used as the color developer, slight color development is unfavorably induced in the preparation of a coating liquid for thermosensitive recording layer. After the thermosensitive recording layer is provided using the above-mentioned coating liquid, the color development gradually proceeds to lower the degree of whiteness in the background portion of the recording material. Namely, the so-called fogging problem is caused. In light of the above-described disadvantages of 4,4xe2x80x2-bisphenol S, the use of 4,4xe2x80x2-bisphenol S as the color developer in the thermosensitive recording material has been considered to be difficult.
It is therefore a first object of the present invention to provide a thermosensitive recording composition free of toxicity and the cause of environmental pollution, which shows high coloring sensitivity without causing any color development at the step of preparing a coating liquid or any phenomenon of fogging on the background after the coating liquid is coated.
A second object of the present invention is to provide a remarkably useful thermosensitive recording material.
The first object of the present invention can be achieved by a thermosensitive recording composition comprising a leuco dye, a color developer comprising 4,4xe2x80x2-bisphenol S, and a sensitizer comprising 4-acetylbiphenyl.
The second object of the present invention can be achieved by a thermosensitive recording material comprising a support, and a thermosensitive recording layer which is provided on the support and comprises a thermosensitive recording composition comprising a leuco dye, a color developer comprising 4,4xe2x80x2-bisphenol S, and a sensitizer comprising 4-acetylbiphenyl.